1. Technical Field
The present invention relates to an acoustic transducer. More specifically, the present invention relates to a small-sized acoustic transducer for an acoustic sensor (microphone), a speaker, or the like manufactured by utilizing a MEMS technology.
2. Related Art
FIG. 1 is a schematic sectional view illustrating a general capacitance type acoustic sensor 11. In the acoustic sensor 11, a diaphragm 15 (vibration electrode plate) and a fixed electrode plate 19 that face each other at an interval are disposed on the upper surface of a silicon substrate 12. The silicon substrate 12 is provided with a chamber 13 (hollow) that vertically penetrates, and the diaphragm 15 covers an opening 14 of the chamber 13 in the upper surface of the silicon substrate 12. The outer edge of the diaphragm 15 faces the upper surface of the silicon substrate 12 with a narrow gap sandwiched therebetween. This gap is a vent hole 16 for communicating the upper surface side of the diaphragm 15 with the lower surface side. An insulating protective film 18 provided on the upper surface of the silicon substrate 12 covers the whole of the diaphragm 15. The fixed electrode plate 19 is provided on the lower surface of the protective film 18 so as to face the diaphragm 15. When the diaphragm 15 vibrates by acoustic vibration, electrostatic capacitance value between the diaphragm 15 and the fixed electrode plate 19 changes, the acoustic vibration is converted into an electric signal.
In the capacitance type acoustic sensor 11 as illustrated in FIG. 1, in order to prevent the diaphragm 15 from sticking to and disabling to separate from the fixed electrode plate 19, stoppers 20 provided in the protective film 18 protrude from the fixed electrode plate 19. The stoppers 20 are provided, so that when the diaphragm 15 is largely displaced, the diaphragm 15 first hits the stoppers 20, and the diaphragm 15 comes into contact with the fixed electrode plate 19, thereby preventing the sticking.
Furthermore, when the diaphragm 15 is largely displaced, the diaphragm 15 presses against the silicon substrate 12, thereby causing a mute phenomenon, or a phenomenon in which the diaphragm 15 sticks to the upper surface of the silicon substrate 12 and does not separate from the upper surface of the silicon substrate 12. There is known an acoustic sensor provided with projections 17 on the lower surface of the outer edge of a diaphragm 15. Therese projections 17 aim to obtain effects similar to the stoppers 20. The acoustic sensor provided with such projections on the lower surface of the diaphragm is disclosed in, for example, Patent Document 1.
The sticking is a phenomenon in which the diaphragm sticks to the fixed electrode plate, the substrate upper surface, or the like, and does not separate from the fixed electrode plate, the substrate upper surface, or the like, which is caused by the large displacement of the diaphragm and the collision with the fixed electrode plate or the substrate, or also caused by surface tension of water which enters between the diaphragm and the fixed electrode plate or the substrate upper surface in a step of manufacturing the acoustic sensor.
FIG. 2 is a schematic plan view illustrating a state where the protective film 18 and the fixed electrode plate 19 are removed from the above acoustic sensor 11, and illustrates the arrangement of the projections 17 provided on the lower surface of the diaphragm 15. (The projections are provided on the lower surface of the diaphragm, and therefore the projections are not seen from the plan view, but are illustrated solid lines for convenience. The same applies hereafter.) FIG. 3(A) is an enlarged plan view of a part of K1 in FIG. 2, and FIG. 3(B) is a sectional view taken along the line K2-K2 of FIG. 3(A). (A structure seen across the section is also illustrated in the sectional view. The same applies hereafter.) The diaphragm 15 extends beam portions 21 in diagonal directions from four corners, and each beam portion 21 is fixed on the anchor (not illustrated) provided on the upper surface of the silicon substrate 12. The diaphragm 15 has a shape almost analogous to the opening 14 except the beam portions 21, and a region to overlap with the upper surface of the silicon substrate 12 (region around the opening 14) in the lower surface of the diaphragm 15 (hereinafter, referred to as the outer edge of the diaphragm 15) has an almost uniform width.
On the lower surface of the outer edge of the diaphragm 15, the projections 17 are almost uniformly distributed. As illustrated in FIG. 3(A), the projections 17 are arranged in a plurality of rows in parallel to the edge 14A of the opening 14. That is, the projections 17 are arranged at proper intervals along a straight line a parallel to the edge 14A (outer periphery) of the opening 14, in each side of the outer edge of the diaphragm 15. In an example illustrated in the drawings, the projections 17 are arranged in three rows.
The action of the projections 17 is to touch the upper surface of the silicon substrate 12 in order to prevent the diaphragm 15 from being brought into contact with and sticking to the upper surface of the silicon substrate 12 when the diaphragm 15 is largely deformed. However, when the arrangement density (number density) of the projections 17 is increased, the total area of contact surfaces between the projections 17 and silicon substrate 12 is increased as illustrated in FIG. 4(A) and FIG. 4(B). (The number of rows of the projections 17 are increased to four rows in FIG. 4(A) and FIG. 4(B).) Therefore, adhesive strength between the projections 17 and the silicon substrate 12 is larger than force for elastically returning the diaphragm 15 upward, thereby causing a defect that the projections 17 themselves stick to the silicon substrate 12.
Thus, the arrangement density (or the number of) of the projections 17 is restricted, and therefore a distance Ledge from the edge 14A of the opening 14 to the projection 17 located closest to the edge 14A is also likely to increases. In a case where this distance Ledge is long, there is a risk that a mute phenomenon occurs. The mute phenomenon is a phenomenon that the acoustic sensor 11 does not pick up sound (i.e., does not detect acoustic vibration). For example, in a microphone module 22 in which a package 23 houses the acoustic sensor 11 and the processing circuit 24 as illustrated in FIG. 6, a case of strongly blowing from a sound introduction hole 26 into the microphone module 22 is considered. The breath blown against the sound introduction hole 26 flows to a package internal space 25 (back chamber) through the chamber 13 (front chamber) of the acoustic sensor 11, and therefore the pressure of the package internal space 25 instantaneously rises right after strongly blowing on the package internal space 25. A dotted patterned part in FIG. 6 indicates a high pressure region. On the other hand, the inside of the chamber 13 of the acoustic sensor 11 rapidly returns to the atmospheric pressure. Therefore, as illustrated in FIGS. 5(A) and 5(B), the diaphragm 15 is strongly pressed toward the substrate by the pressure of the package internal space 25, to close the opening 14 of the chamber 13, so that sound is not picked up until the package internal space 25 returns to the atmospheric pressure. Thus, the mute phenomenon occurs.
In the vicinity of the opening 14, when the diaphragm 15 is regarded as a cantilever supported by the projections 17 located on the end, the displacement of the cantilever (displacement of the diaphragm 15 at the edge 14A of the opening 14) is proportional to Ledge3. Accordingly, in order that the mute phenomenon is unlikely to occur, the distance Ledge from the edge 14A of the opening 14 to each projection 17 located nearest to the edge 14A needs to reduce as short as possible. In order to shorten the distance Ledge, as illustrated in FIG. 7(A), the whole of the projections 17 is simply moved in parallel toward the edge of the opening 14. However, in this method, a distance Louter from the edge 15A of the diaphragm 15 to each projection 17 located nearest to the edge 15A increases, and therefore the vicinity of the edge 15A of the diaphragm 15 is likely to stick to the silicon substrate 12, as illustrated in FIG. 7(B).
In order to prevent the mute phenomenon or the sticking in the vicinity of the edge 15A of the diaphragm 15, it is considered that the projections 17 located in a row close to the edge 14A of the opening 14 are moved in parallel toward the edge of the opening 14, and the projections 17 located in a row close to the edge 15A of the diaphragm 15 are moved in parallel toward the edge of the diaphragm 15, as illustrated in FIG. 8(A). However, in this case, intervals Lint between the projections 17 are widened, and therefore a risk that parts located between the rows of the projections 17 are bent to stick to the upper surface of the silicon substrate 12 increases as illustrated in FIG. 8(B).
It is considered that the width W of the outer edge of the diaphragm 15 (length of the vent hole 16) is shortened without changing the intervals between the rows of the projections 17. When the width W of the outer edge of the diaphragm 15 is shortened, both the distances Ledge and Louter can be reduced. However, in this case, the width W of the diaphragm 15 (length of the vent hole 16) is shortened, and therefore acoustic resistance in the vent hole 16 is reduced, sensitivity in low-pitched sound range is lowered, and the frequency characteristic of the acoustic sensor is lowered.
As another method of preventing the mute phenomenon, it is considered that the heights of the projections 17 are increased, as illustrated in FIG. 9(A). In order to close the opening 14 by the diaphragm 15, deformation equivalent to at least the heights of the projections 17 is required. Therefore, the higher the projections 17 become, the less frequently the mute phenomenon occurs. However, when the heights of the projections 17 are increased, a distance d between the lower surface of each projection 17 and the upper surface of the silicon substrate 12 is shortened, and therefore the projections 17 are likely to stick to the silicon substrate 12.
Furthermore, as illustrated in FIG. 9(B), in a case where the heights of the projections 17 are increased, and the distance between the lower surface of each projection 17 and the upper surface of the silicon substrate 12 is increased, a distance H between the lower surface of the diaphragm 15 and the upper surface of the silicon substrate 12 (height of the vent hole 16) is increased. This causes a defect that acoustic resistance in the vent hole 16 is reduced, sensitivity in low-pitched sound range is lowered, and the frequency characteristic of the acoustic sensor is lowered.
The projections 17 are uniformly arranged at the outer edge of the diaphragm 15. This is because when the arrangement of the projections 17 is nonuniform and density is uneven, the sticking to the substrate as described above is likely to occur. On the other hand, according to one or more embodiments of the present invention, from the point of the view of noise design, the width W of the outer edge of the diaphragm 15 is constant. Accordingly, the diaphragm 15 needs to have a shape almost analogous to the opening 14. For example, according to one or more embodiments of the present invention, in a case where the opening 14 is circular, the diaphragm 15 is circular, as illustrated in FIG. 10(A).
FIG. 10(A) illustrates a conventional acoustic sensor in which a circular diaphragm 15 is provided on a silicon substrate 12 having a circular opening 14 of a chamber. Additionally, FIG. 10(B) is an enlarged plan view of a part of K3 in FIG. 10(A). Such a conventional example, the projections 17 are provided along arcs b (concentric circles) parallel to the edge 14A of the opening 14 and the edge 15A of the diaphragm 15. In order to make the arrangement density of the projections 17 uniform over the whole outer edge of the diaphragm 15, the projections 17 are generally provided along the arcs b (concentric circular) parallel to the edge 14A of the opening 14, as illustrated in FIG. 10(A) and FIG. 10(B).
Patent Document 1: International Publication No. 2002/015636 (WO2002/015636A)